Question

a nova occurs when hydrogen accreted onto a white dwarf from a close companion rapidly fuses to helium, releasing a large amount of energy. a massive star experiences collapse of its iron core and explodes. a white dwarf accretes enough material that it has a mass greater than 1.4 solar masses, collapses, and explodes. binary black holes radiate energy away as gravity waves, spiral in, and collide.

Explanation:

A nova is caused by hydrogen from a close - companion star accreting onto a white dwarf and rapidly fusing to helium, releasing a large amount of energy. The other options describe different astrophysical events: a massive star's iron - core collapse and explosion is a supernova; a white dwarf with a mass greater than 1.4 solar masses collapsing and exploding is also a type of supernova (Type Ia); and binary black holes radiating energy as gravity waves and colliding is a different gravitational - wave source event.

Answer:

A. hydrogen accreted onto a white dwarf from a close companion rapidly fuses to helium, releasing a large amount of energy.